Method of making high critical temperature metal nitride layer

ABSTRACT

A method of fabricating a device including a superconductive layer includes depositing a seed layer on a substrate at a first temperature, the seed layer being a nitride of a first metal, reducing the temperature of the substrate to a second temperature that is lower than the first temperature, increasing the temperature of the substrate to a third temperature that is higher than the first temperature to form a modified seed layer, and depositing a metal nitride superconductive layer directly on the modified seed layer at the third temperature, the superconductive layer being a nitride of a different second metal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Application Ser. No.62/980,101, filed on Feb. 21, 2020, the entire disclosure of which isincorporated by reference.

BACKGROUND Technical Field

The disclosure concerns use of a seed layer to improve thesuperconducting critical temperature of a metal nitride layer.

Background Discussion

In the context of superconductivity, the critical temperature (Tc)refers to the temperature below which a material becomessuperconductive. Niobium nitride (NbN) is a material that can be usedfor superconducting applications, e.g., superconducting nanowire singlephoton detectors (SNSPD) for use in quantum information processing,defect analysis in CMOS, LIDAR, etc. The critical temperature of niobiumnitride depends on the crystalline structure and atomic ratio of thematerial. For example, referring to FIG. 1, cubic δ-phase NbN has someadvantages due to its relatively “high” critical temperature, e.g.,9.7-16.5 K (the indicated process temperatures are for a particularfabrication process, and not necessarily applicable other process anddeposition chamber designs).

Niobium nitride can be deposited on a workpiece by physical vapordeposition (PVD). For example, a sputtering operation can be performedusing a niobium target in the presence of nitrogen gas. The sputteringcan be performed by inducing a plasma in the reactor chamber thatcontains the target and the workpiece.

SUMMARY

In one aspect, a method of fabricating a device including asuperconductive layer includes depositing a seed layer on a substrate,exposing the seed layer to an oxygen-containing gas or plasma to form amodified seed layer, and after exposing the seed layer to theoxygen-containing gas or plasma depositing a metal nitridesuperconductive layer directly on the modified seed layer. The seedlayer is a nitride of a first metal, and the superconductive layer is anitride of a different second metal.

In another aspect, a method of fabricating a device including asuperconductive layer includes depositing a lower seed layer on asubstrate, depositing an upper seed layer directly on the lower seedlayer, and depositing a metal nitride superconductive layer directly onthe upper seed layer. The lower seed layer is a nitride of a firstmetal, the upper seed layer is an oxide or oxynitride of the firstmetal, and the superconductive layer is a nitride of a different secondmetal.

In another aspect, a method of fabricating a device including asuperconductive layer includes depositing a seed layer on a substrate,and depositing a metal nitride superconductive layer directly on theseed layer. The seed layer is an oxide or oxynitride of a first metal,and the superconductive layer is a nitride of a different second metal.

In another aspect, a method of fabricating a device including asuperconductive layer includes depositing a seed layer on a substrate ata first temperature, reducing the temperature of the substrate to asecond temperature that is lower than the first temperature, increasingthe temperature of the substrate to a third temperature that is higherthan the first temperature to form a modified seed layer, and depositinga metal nitride superconductive layer directly on the modified seedlayer at the third temperature. The seed layer is a nitride of a firstmetal, and the superconductive layer is a nitride of a different secondmetal.

Implementations may provide, but are not limited to, one or more of thefollowing advantages. The critical temperature of the metal nitridelayer, e.g., the NbN layer, can be increased. This permits fabricationof devices, e.g., SNSPDs, with superconductive wires with a highercritical temperature. The larger difference between the operatingtemperature (2-3 K) and the critical temperature provides superiordetection efficiency, lower dark count, and possibly faster temporalresponse.

It should be noted that “superconductive” indicates that the materialbecomes superconducting at the operating temperature of the device,e.g., 2-3° K. The material is not actually superconducting duringfabrication of the device at or above room temperature or when thedevice is not being cooled for operation.

The details of one or more implementations are set forth in theaccompanying drawings and the description below. Other potentialaspects, features, and advantages will become apparent from thedescription, the drawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 diagram illustrating phase of niobium nitride as a function ofprocessing temperature and atomic percentage nitrogen.

FIG. 2A is a schematic cross-sectional view of a device that includes ametal nitride lower seed layer, a metal oxide or oxynitride upper seedlayer, and a superconductive metal nitride layer.

FIG. 2B is a schematic cross-sectional view of the device of FIG. 2A inwhich the superconductive layer has been etched to form superconductivewires.

FIGS. 3A-3C are flow charts of a method of fabricating the device ofFIG. 2A or 2B.

FIG. 4A is a schematic cross-sectional view of a device that includes ametal oxide or oxynitride seed layer and a superconductive metal nitridelayer.

FIG. 4B is a schematic cross-sectional view of the device of FIG. 4A inwhich the superconductive layer has been etched to form superconductivewires.

FIG. 5 is a flow chart of a method of fabricating the device of FIG. 4Aor 4B.

FIG. 6A is a schematic cross-sectional view of a device that includes ametal nitride seed layer and a superconductive metal nitride layer.

FIG. 6B is a schematic cross-sectional view of the device of FIG. 6A inwhich the superconductive layer has been etched to form superconductivewires.

FIG. 7 is a flow chart of a method of fabricating the device of FIG. 6Aor 6B.

FIG. 8A is a schematic top view of a SNSPD that includes a distributedBragg reflector.

FIG. 8B is a schematic cross-sectional side view of the device of FIG.8A.

FIG. 9A is a schematic top view of a SNSPD that includes a waveguide.

FIG. 9B is a schematic cross-sectional side view of the device of FIG.9A.

FIG. 9C is a schematic cross-sectional side view of anotherimplementation of the device of FIG. 9A.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

As noted above, niobium nitride, particularly δ-phase NbN, has someadvantages as a superconductive material. However, δ-phase NbN can bedifficult to deposit at a satisfactory quality. Moreover, the largerdifference between the operating temperature (2-3 K) and the criticaltemperature, the better the device performance. An aluminum nitride(AlN) layer can be used as a seed layer to improve the criticaltemperature of the NbN layer. Without being limited to any particulartheory, the AlN seed layer may induce a crystalline structure in the NbNlayer that provides an increased critical temperature.

However, it has been surprisingly discovered that exposure of the AlNseed layer to atmosphere and at room temperature before deposition ofthe NbN layer can actually provide a higher critical temperature, e.g.,by about 0.5 K, than performing deposition of the NbN layer on the AlNseed layer without breaking vacuum and reducing the substratetemperature. Again, without being limited to any particular theory, twonon-exclusive possibilities have been proposed. First, exposure of theAlN to atmosphere may result in the formation of a thin aluminum oxideor aluminum oxynitride layer on the surface of the AlN layer, whichinduces a superior crystalline structure in the NbN layer. Second,thermally cycling the AlN seed layer by reducing the temperature of thesubstrate from a first deposition temperature for the AlN, e.g., 400°C., to room temperature, i.e., 20-22° C., and then raising thetemperature of the substrate back to a second deposition temperature forthe NbN, e.g., 400° C., may affect stress in the AlN seed layer whichmay affect its crystalline structure, which in turn can affect thecrystalline structure of the NbN layer.

FIG. 2A is a schematic illustration of some layers in a device 100 thatincludes a metal nitride layer 108 for use as a superconductivematerial. FIG. 2B is a schematic illustration of a device 100 in whichthe metal nitride layer has been formed into features, e.g.,superconductive wires 108′. The device 100 could be superconductingnanowire single photon detectors (SNSPD), a superconducting quantuminterference device (SQUID), a circuit, e.g., an RF line, in a quantumcomputer, etc. FIGS. 3A-3C are flowcharts of methods 200 of fabrication.

The metal nitride layer 108 is disposed on a support structure 102. Thesupport structure 102 can include a substrate, e.g., a silicon wafer.The substrate can be a dielectric material, e.g., sapphire, SiO₂, fusedsilica, or quartz, or a semiconductor material, e.g., silicon, galliumnitride (GaN) or gallium arsenide (GaAs). Although illustrated as asingle block, the support structure 102 could include multipleunderlying layers. For example, the support structure 102 can include adistributed Bragg reflector (DBR) that includes multiple pairs of layersformed of high refractive index and low refractive index materialsdeposited over the substrate, or a waveguide formed on the substrate.

A seed layer structure 103 is formed over the support structure 102. Theseed layer structure 103 includes a lower layer seed layer 104 and anupper layer seed layer 106.

Covering the top of the support structure 102, e.g., in direct contactwith the top surface of the support structure 102, is a lower layer seedlayer 104. The lower seed layer 104 is a metal nitride layer. Inparticular, the lower seed layer 104 and the superconductive layer 108are nitrides of different metals. The lower seed layer 104 can bealuminum nitride (AlN). However, hafnium nitride (HfN), chromium nitride(CrN), or nitride of an alloy of aluminum and either hafnium orscandium, might also be suitable.

The lower seed layer 104 can have a thickness of about 3 to 50 nm, e.g.,about 5 nm or about 10 nm or about 20 nm thickness. The lower seed layer104 can have a (002) c-axis crystal orientation. The lower seed layer104 need not be superconducting at the operating temperature of thedevice 100. The lower seed layer 104 can be deposited (step 202) by astandard chemical vapor deposition or physical vapor deposition process.The deposition process can be conducted with the substrate at atemperature of 200-500° C., e.g., 400° C.

Exemplary processing parameters for the lower seed layer are a powerapplied to the sputtering target of 1-5 kW, a total pressure (nitrogenand inert gas) of 2 to 20 mTorr with nitrogen gas and inert gas suppliedat a ratio between 3:100 and 6:1, e.g., about 3:1, a wafer temperatureof 200-500° C., and no bias voltage applied to the wafer.

Formed on top of the lower seed layer 104, e.g., in direct contact withthe top surface of the lower seed layer 104, is an upper seed layer 106(step 204). The upper seed layer 106 is a metal oxide or metaloxynitride layer. In particular, the upper seed layer 106 is an oxide oroxynitride of the same metal as the metal of the metal nitride in thelower seed layer 104. The upper seed layer 106 can be aluminum oxide oraluminum oxynitride, as this appears to improve the critical temperatureof NbN, e.g., by about 0.5 K over aluminum nitride as a seed layer.However, hafnium oxide or oxynitride, chromium oxide or oxynitride, or anitride or oxynitride of an alloy of aluminum and either hafnium orscandium, might also be suitable.

The upper seed layer 106 can be thinner than the lower seed layer 104.The upper seed layer 106 can be about 0.1-3 nm thick, depending on themethod of fabrication. In some implementations, the upper seed layer 106is only one to five atomic layers thick, e.g., two or three atomiclayers thick. The upper seed layer 106 can have a (002) c-axis crystalorientation. The upper seed layer 106 need not be superconducting at theoperating temperature of the device 100.

Referring to FIG. 3A, one technique that can be used to form the metaloxide or metal oxynitride of the upper seed layer 106 is to expose thelower seed layer 104 to a gas containing oxygen and/or water (step 204a). For example, the lower seed layer 104 can be exposed to air. Asanother example, the lower seed layer 104 could be exposed to pureoxygen. As another example, the lower seed layer 104 could be exposed toa gas mixture containing oxygen at 20-90% by volume and one or moreother gases, such nitrogen and/or a noble gas, e.g., argon. In someimplementations, the gas mixture includes water, e.g., water vapor orsteam. The pressure can be 1 Torr to 1 atmosphere, e.g., 0.8 to 1atmosphere.

Referring to FIG. 3B, another technique that can be used to form themetal oxide or metal oxynitride of the upper seed layer 106 is to exposethe lower seed layer 104 to a gas containing oxygen (O₂) plasma (step204 b). For example, the lower seed layer 104 could be exposed to pureoxygen plasma. For example, oxygen gas can be directed into a plasmaprocessing chamber, and an oxygen plasma can be formed at a power ofabout 100 W. The pressure can be 2 to 500 mTorr. In general, a dedicatedchamber for oxygen plasma treatment can use a relatively higherpressure, e.g., 100-500 mTorr, whereas a relatively lower pressure,e.g., 2 to 15 mTorr, can be used if the oxygen plasma treatment isperformed in the same chamber that is used for the deposition of thelower seed layer.

Without being limited to any particular theory, exposure of AlN tooxygen may result in the formation of a thermal oxide or thermaloxynitride layer, i.e., an aluminum oxide or aluminum oxynitride layer,on the surface of the AlN layer.

In some implementations, the substrate with the lower seed layer islowered from a first temperature at which the lower seed layer isdeposited, e.g., 300-500° C., to a lower second temperature, e.g.,20-300° C. The lower seed layer is exposed to the oxygen-containing gasor plasma at the lower second temperature. The second temperature can beat least 200° C. lower than the first temperature. For example, thesecond temperature can be room temperature, i.e., 20-22° C. Thesubstrate is then raised to an elevated third temperature for depositionof the metal nitride of the superconductive layer.

In some implementations, the substrate with the lower seed layer ismaintained at an elevated temperature, e.g., at or above 300° C., e.g.,at the same temperature at which the lower seed layer is deposited,e.g., 400° C., and the substrate is exposed to the oxygen-containing gasor plasma at the elevated temperature.

In some implementations, the substrate with the lower seed layer islowered from the first temperature to the second temperature, thenraised up to elevated third temperature, e.g., at or above 300° C.,e.g., 300-500° C., and the lower seed layer exposed to theoxygen-containing gas or plasma, at the elevated third temperature.

The exposure time can depend on pressure and temperature, and can befrom 1 second to 120 minutes. For example, the exposure time foratmosphere at room temperature can be about 45 minutes. As anotherexample, the exposure time for oxygen plasma with the substrate at thesame temperature at which the lower seed layer is deposited, e.g., atabout 400° C., can be about 30 seconds.

In the techniques of FIGS. 3A and 3B, the upper seed layer 106 iseffectively a native oxide or native oxynitride formed on the underlyingmetal nitride layer, and thus expected to be two to four atomic layersthick. For example, the upper seed layer 106 can be up to about 1 nmthick.

Referring to FIG. 3C, another technique that can be used to form themetal oxide or metal oxynitride of the upper seed layer 106 is todeposit the upper seed layer 106 by physical vapor deposition. Exemplaryprocessing parameters for the upper seed layer are a power applied tothe sputtering target of 1-5 kW, a total pressure (oxygen and inert gas)of 2 to 20 mTorr with oxygen gas and inert gas supplied at a ratiobetween 3:100 and 6:1, and a wafer temperature of 200-500° C. There arealso CVD and ALD techniques to deposit aluminum oxide or oxynitride.

In the technique of FIG. 3C, the thickness of the upper seed layer 106depends on the processing time or number of iterations of the depositionprocess. For example, the thickness of the upper seed layer 106 can be1-2 nm.

Returning to FIGS. 2A and 2B, the superconductive metal nitride layer108 is deposited (step 206) on, e.g., in direct contact with, the upperseed layer 106. The metal nitride layer 108 is formed of niobium nitride(NbN), titanium nitride (TiN), or niobium titanium nitride(Nb_(X)Ti_(1-X)N). The superconductive layer 108 can have a thickness of4 to 50 nm, e.g., about 5 nm or about 10 nm or about 20 nm.

The metal nitride layer 108 can be deposited using a standard chemicalvapor deposition or physical vapor deposition process. Exemplaryprocessing parameters are a base pressure of 1e-8 Torr, a power appliedto the target of 1-3 kW, a total pressure during processing of 5-7mTorr, a wafer temperature of 400° C., no bias voltage applied to thewafer, and a percentage of the gas as N₂ sufficient to achieve cubicδ-phase NbN. In some implementations, the metal nitride layer 108 isdeposited in the same processing chamber that is used to deposit thelower seed layer 104 and upper seed layer 106, e.g., by switching in anew target. This permits higher throughput manufacturing. Alternatively,the substrate can be transported to a different deposition chamberwithout breaking vacuum. This permits the metal nitride layer to bedeposited without exposure of the seed layer to atmosphere and withlower risk of contamination.

After the metal nitride layer 108 is deposited, a capping layer 110 canbe deposited on the metal nitride layer 108 (step 208). The cappinglayer 110 serves as a protective layer, e.g., to prevent oxidation ofthe metal nitride layer 108 or other types of contamination or damage.The capping layer 108 can be dielectric but need not be superconductiveat the operating temperature of the device 100. The capping layer 108can be amorphous silicon (a-Si). In some implementations, the cappinglayer 108 is a nitride of a different material from the metal of themetal nitride used for the superconductive layer 108. Examples ofmaterials for the capping layer 108 include AlN, Al₂O₃, SiO₂, and SiN.The capping layer 108 can be deposited by a standard chemical vapordeposition or physical vapor deposition process.

Etching can be used to form trenches 112 through at least the metalnitride layer 108 to form the superconductive wires 108′ or otherstructures needed for the device 100 (step 210). The wires 108′ can havea width of about 25 to 250 nm, e.g., about 60 nm. Although FIG. 2Billustrates the trenches 112 as extending through the metal nitridelayer 108 and capping layer 110 and not into the upper seed layer 106,other configurations are possible. As an example, the trenches 112 canextend partially into or entirely through the upper seed layer 106, orentirely through the upper seed layer 106 and partially into or entirelythrough the lower seed layer 104.

Air can contain contaminants, so for any of the above processes, theupper seed layer 106 can be formed on the lower seed layer 104 withoutbreaking vacuum, e.g., without removing the substrate from thedeposition chamber in which the lower seed layer is deposited, orwithout breaking vacuum during transfer of the substrate from thedeposition chamber in which the lower seed layer is deposited to thechamber in which the upper seed layer is formed. Similarly, the metalnitride superconductive layer 108 can be formed on the upper seed layer106 without breaking vacuum.

Where the upper seed layer 106 is formed by oxygen plasma treatment (seeFIG. 3B) or by PVD (see FIG. 3C), an Applied Materials Endura® withImpulse PVD could be used. The deposition of the lower seeding layer andeither oxygen plasma treatment or PVD of an oxide or oxynitride could beperformed within the same chamber. NbN deposition can be performed in adifferent chamber in the same Endura tool, but without breaking vacuum.

FIG. 4A is a schematic illustration of some layers in a device 100′ thatincludes a metal nitride layer 108 for use as a superconductivematerial. FIG. 4B is a schematic illustration of a device 100′ in whichthe metal nitride layer has been formed into features, e.g.,superconductive wires 108′. The device 100′ is similar to the device100, but instead of having both a lower seed layer and an upper seedlayer, the seed layer structure 103 of device 100′ has a single metaloxide or metal oxynitride seed layer 106′. Except as discussed below,the device 100′ can be configured and manufactured as discussed withrespect to device 100. FIG. 5 is a flowchart of a method 200′ offabrication.

A seed layer 106′ is disposed on top of the support structure 102. Theseed layer 106′ is a metal oxide or metal oxynitride. In particular, theseed layer 106′ is an oxide or oxynitride of a different metal than themetal of the metal nitride in the superconductive layer 108. The seedlayer 106′ can be aluminum oxide or aluminum oxynitride (AlN), as thisappears to improve the critical temperature of NbN, e.g., by about 0.5 Kover aluminum nitride as a seed layer. However, hafnium oxide, hafniumoxynitride, gallium oxide, or gallium oxynitride might also be suitable.Unlike the device 100, there is no metal nitride layer of the same metalin direct contact with the bottom of the metal oxide or oxynitride seedlayer 106′.

The seed layer 106′ can have a thickness of about 3 to 50 nm, e.g.,about 5 nm or about 10 nm or about 20 nm thickness. The seed layer 106′can have a (002) c-axis crystal orientation. The seed layer 106′ neednot be superconducting at the operating temperature of the device 100.The seed layer 106′ can be deposited (step 204′) by a standard chemicalvapor deposition or physical vapor deposition process. The depositionprocess can be conducted with the substrate at a temperature of 200-500°C., e.g., 400° C.

Exemplary processing parameters are a power applied to the sputteringtarget of 1-5 kW, a total pressure (nitrogen and inert gas) of 2 to 20mTorr with nitrogen gas and inert gas supplied at a ratio between 3:100and 1:6, a wafer temperature of 200-500° C., and no bias voltage appliedto the wafer.

An Applied Materials Endura® with Impulse PVD could be used fordeposition of the seed layer and the superconductive layer. For example,deposition of aluminum oxide can be performed in a first chamber, andNbN deposition can be performed in a different chamber in the same tool,but without breaking vacuum.

Thermal cycling can be applied between the deposition of the seed layer106′ and the superconductive layer 108. For example, the substrate withthe seed layer 106′ is lowered from the first temperature to the secondtemperature, then raised up to elevated third temperature, e.g., at orabove 300° C., e.g., 300-500° C., for deposition of the metal nitridesuperconductive layer 108. Alternatively, the substrate with the seedlayer 106′ can be maintained at an elevated temperature, e.g., at orabove 300° C., e.g., at the same temperature at which the seed layer106′ is deposited, e.g., 400° C., until deposition of the metal nitridesuperconductive layer 108.

FIG. 6A is a schematic illustration of some layers in a device 100″ thatincludes a metal nitride layer 108 for use as a superconductivematerial. FIG. 6B is a schematic illustration of a device 100″ in whichthe metal nitride layer has been formed into features, e.g.,superconductive wires 108′. The device 100″ is similar to the device100′, but instead of having a seed layer of metal oxide or metaloxynitride, the seed layer structure 103 of device 100″ includes asingle layer of metal nitride that has been subjected to thermalcycling. Except as discussed below, the device 100 can be configured andmanufactured as discussed with respect to devices 100 and 100′. FIG. 7is a flowchart of a method 200″ of fabrication.

A seed layer 104′ is disposed on top of the support structure 102. Theseed layer 104′ is a metal nitride. In particular, the seed layer 104′and the superconductive layer 108 are nitrides of different metals. Theseed layer 104′ can be aluminum nitride. However, hafnium nitride orgallium nitride might also be suitable. Unlike the device 100, there isno metal oxide or metal oxynitride between the seed layer 104′ and thesuperconductive layer 108.

The seed layer 104′ can be deposited (step 204′) directly on the supportstructure 102 by a standard chemical vapor deposition or physical vapordeposition process. The deposition process can be conducted with thesubstrate at a first temperature of 200-500° C., e.g., 400° C.

After deposition, the substrate with the metal nitride seed layer issubjected to thermal cycling (step 205). In particular, the substratewith the seed layer is lowered from the first temperature at which theseed layer is deposited, e.g., 200-500° C., to a lower secondtemperature. For example, the substrate with the seed layer 104′ islowered from the first temperature at which the seed layer is deposited,e.g., 300-500° C., to a lower second temperature, e.g., 20-300° C. Thesecond temperature can be at least 200° C. lower than the firsttemperature. For example, the second temperature can be roomtemperature, i.e., 20-22° C. The seed layer can be subject to thermalcycling while in vacuum, or while exposed to nitrogen and/or an inertgas, e.g., argon. The substrate is then raised to an elevated thirdtemperature, e.g., e.g., 300-500° C., for deposition of the metalnitride of the superconductive layer. Thermally cycling may change thecrystalline structure of the seed layer 104′.

After the thermal cycling, the metal nitride of the superconductivelayer 108 can be deposited on the seed layer 104′. The superconductivelayer 108 is deposited without breaking vacuum or otherwise exposing theseed layer to oxygen or an oxygen-containing vapor, e.g., H₂O.

FIGS. 8A and 8B illustrate top and side views, respectively, of a device100 a configured as a superconducting nanowire single photon detector(SNSPD). The device 100 a can use any configuration of the seed layer103 discussed above.

The SNSPD device 100 a can include at least one superconductive wire108′ disposed on a support structure 102. The superconductive wire 108′can be connected between conductive electrodes 120. The superconductivewire 108′ can be arranged in a meandering pattern, e.g., aback-and-forth parallel lines, on the supporting structure 102. In someimplementations, multiple wires 108′ are connected in parallel betweenthe electrodes 120, with each wire 108′ covering a separate area 152,but there could be just a single wire 108′ covering the entire detectionarea of the device 100 a. In addition, many other patterns are possible,e.g., zigzag or double spiral.

The support structure 102 includes a substrate 124 and a distributedBragg reflector (DBR) 126 that includes multiple pairs of layers formedof high refractive index and low refractive index materials.

The SNSPD device 100 a is operated with a photon (illustrated by lightbeam 10) approaching from the top of the device 100 a, e.g., with normalincidence relative to the substrate 124. The working principle of theSNSPD device is that the to-be-detected photon comes from top and shineson the SNPSD. Absorption of the photon, either on initial impingement orupon reflection from the DBR, creates a hot spot on the NbN nanowirewhich raises the temperature of the NbN above critical temperature sothat a portion of the wire is no longer in the superconductive state. Aregion around the hot spot can experience current crowding, resulting ina higher current density than the critical current density, which candisrupt the superconductive state for the entire wire. The change in theNbN wire from the superconducting state to the normal resistive statecan be electrically detected by flowing a current through the device andmonitoring voltage differences between the electrodes.

Another form of superconducting nanowire single photon detector (SNSPD)device includes a waveguide to input photons into the detector along anaxis generally parallel to the surface of the substrate. FIGS. 9A and 9Billustrate a device 100 b configured as a superconducting nanowiresingle photon detector (SNSPD) and having a waveguide 138. The device100 b can use any configuration of the seed layer 103 discussed above.

The SNSPD device 100 b can include at least one superconductive wire108′ disposed on a support structure 102. The superconductive wire(s)108′ can be arranged to form a plurality of parallel lines, withadjacent lines connected at alternating ends. Although FIG. 9Aillustrates four parallel lines, the device could have just two parallellines, e.g., a U-shaped wire, or a greater number of lines. Thesuperconductive wire 108′ can be connected between conductiveelectrodes.

The support structure 102 can include a substrate 134, a dielectriclayer 136 on the substrate 134, and a waveguide 138 disposed on thedielectric layer 136. The dielectric layer 102 c is a first materialhaving a first refractive index, and the waveguide 102 d is a secondmaterial having a second refractive index that is higher than the firstrefractive index. Photons, shown by light beam 10 b, are injected intothe device from the side, e.g., generally parallel to the top surface ofthe substrate 132, through the waveguide 138. In particular, the photonscan enter along an axis (shown by arrow A) generally parallel to theparallel lines of the wire 108′.

In addition, along the axis transverse to the direction of lightpropagation, the wire 108′ can be located near the center of thewaveguide 138. For example, on each side of device, there can be a gap130 between the outer edge of the wire 108′ and the outer edge of thewaveguide 138. This gap 130 can have a width of about 25-30% of thetotal width of the waveguide.

In general, because the dielectric layer 136 below the waveguide 138 andthe empty space or air above the waveguide 138 both have a lowerrefractive index than the waveguide 138, the photons in the waveguide138 are trapped by total internal reflection. However, due to theoptical coupling between the waveguide 138 and the nanowire 108′, thephotons can escape into the nanowire 108′ and thus be absorbed by thenanowire 108′. The light coupling efficiency can be very high in thistype of device.

Referring to FIG. 9C, if the waveguide 138 is formed of an appropriatemetal nitride, e.g., aluminum nitride, then the top surface of thewaveguide 138 can provide the lower seed layer and can be treated toform the upper seed layer 106 or the upper seed layer 106 can be formeddirectly on the waveguide 108, i.e., without having to deposit aseparate lower seed layer.

While particular implementations have been described, other and furtherimplementations may be devised without departing from the basic scope ofthis disclosure. It is contemplated that elements and features of oneembodiment may be beneficially incorporated in other embodiments withoutfurther recitation. It is to be noted, however, that the drawingsillustrate only exemplary embodiments. The scope of the invention isdetermined by the claims that follow.

What is claimed is:
 1. A method of fabricating a device including asuperconductive layer, the method, comprising: depositing a seed layeron a substrate, the seed layer being an oxide or oxynitride of a firstmetal; and depositing a metal nitride superconductive layer directly onthe seed layer, the superconductive layer being a nitride of a differentsecond metal.
 2. The method of claim 1, wherein the second metal isniobium nitride, titanium nitride, or niobium titanium nitride.
 3. Themethod of claim 2, wherein the first metal is aluminum.
 4. The method ofclaim 1, wherein the first metal is aluminum.
 5. The method of claim 1,wherein depositing the upper seed layer comprises depositing a layerhaving a thickness of 3-50 nm.
 6. The method of claim 1, whereindepositing the seed layer and depositing the metal nitridesuperconductive layer are performed in a deposition tool withoutbreaking vacuum.
 7. The method of claim 6, wherein depositing the seedlayer and depositing the metal nitride superconductive layer comprisephysical vapor deposition processes.
 8. The method of claim 1, whereindepositing the seed layer and depositing the metal nitridesuperconductive layer comprise physical vapor deposition processes.
 9. Amethod of fabricating a device including a superconductive layer, themethod, comprising: depositing a seed layer on a substrate at a firsttemperature, the seed layer being a nitride of a first metal; reducingthe temperature of the substrate to a second temperature that is lowerthan the first temperature; increasing the temperature of the substrateto a third temperature that is higher than the first temperature to forma modified seed layer; and depositing a metal nitride superconductivelayer directly on the modified seed layer at the third temperature, thesuperconductive layer being a nitride of a different second metal. 10.The method of claim 9, wherein the second metal is niobium nitride,titanium nitride, or niobium titanium nitride.
 11. The method of claim10, wherein the first metal is aluminum.
 12. The method of claim 9,wherein the first metal is aluminum.
 13. The method of claim 9,comprising exposing the seed layer to an oxygen-containing gas or plasmabefore depositing the metal nitride superconductive layer.
 14. Themethod of claim 13, wherein exposing the seed layer to theoxygen-containing gas or plasma is performed with the substrate at thesecond temperature.
 15. The method of claim 13, wherein exposing theseed layer to the oxygen-containing gas or plasma is performed with thesubstrate at the first or third temperature.
 16. The method of claim 9,comprising maintaining the seed layer in an oxygen-free environment fromdepositing the seed layer to depositing the metal nitridesuperconductive layer.
 17. The method of claim 9, wherein secondtemperature is at least 200° C. lower than the first temperature. 18.The method of any of claim 17, wherein second temperature is roomtemperature.
 19. The method of any of claim 9, wherein secondtemperature is room temperature.